OPK Surge Predictor
Enter your recent OPK readings and cycle details to predict when your LH surge will peak and know exactly when to test next.
Your cycle
This helps us estimate when your LH surge is likely.
From day 1 of one period to day 1 of the next.
Day 1 = first day of your current period.
Your OPK results
Log your last few tests — up to 5 days in a row. For each one, enter which cycle day you tested and how dark the line looked.
Test 1
No visible test line
Your data never leaves your device — all calculations run locally in your browser.
Your surge hasn't started yet. Based on your cycle, expect it around day 12–13 of your cycle. Keep testing daily.
Expected peak around day 13 · Expected ovulation around day 14 of your cycle
Save time each day
Analyse your OPK strips instantly with the ClearLine app — no more squinting at faint lines.
Add one more day's test result to see your LH curve chart.
What to do next
When to test
Test once daily, preferably in the late morning or early afternoon.
Testing frequency
Consider testing twice daily once lines start darkening to avoid missing your surge.
What to look for
A test line that is equal to or darker than the control line.
Related calculators
This tool provides estimates based on average cycle patterns and self-reported line strength. Actual LH surge timing varies. This is not a medical diagnostic tool.
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What Is an LH Surge?
Luteinizing hormone (LH) is a key reproductive hormone produced by the pituitary gland. During most of the menstrual cycle, LH levels remain relatively low. However, approximately 24–36 hours before ovulation, LH levels spike dramatically — this is the LH surge.
The LH surge triggers the mature follicle in the ovary to release its egg. This is the biological event that ovulation predictor kits (OPKs) are designed to detect. When LH surges, it appears in your urine within a few hours, causing the test line on an OPK strip to darken until it matches or exceeds the control line.
Timing intercourse around the LH surge maximises your chances of conception. Sperm can survive in the reproductive tract for up to five days, so having sperm present when the egg is released — triggered by the LH surge — gives you the best opportunity to conceive in any given cycle.
How Long Does an LH Surge Last?
An LH surge typically lasts 24–48 hours, though the peak window — when the test line is at its darkest — may only last 12–24 hours. This relatively brief window is why testing frequency matters so much when tracking ovulation with OPKs.
Because the surge can begin and peak within a single day, testing once daily carries a real risk of missing your peak entirely. Many people trying to conceive (TTC) switch to twice-daily testing once they notice their test lines starting to darken — usually testing in the late morning and again in the early evening.
After the surge peaks, LH levels drop quickly. A fading test line usually means ovulation is occurring or has just occurred. The entire surge event — from first detectable rise to return to baseline — typically spans 2–4 days, but the critical peak window is much shorter.
How to Read OPK Results Accurately
Reading OPK results accurately is harder than it sounds. Unlike pregnancy tests where any visible line is significant, OPKs require you to compare the test line against the control line — and subtle differences in darkness are clinically meaningful.
A negative OPK shows a faint or absent test line. A positive OPK shows a test line that is equal to or darker than the control line. The challenge is the grey area in between: a line that is clearly visible but lighter than the control line means your LH is rising but has not yet peaked.
Different brands produce different line colours and intensities. Clearblue strips tend toward blue lines, Easy@Home and Wondfo use pink/magenta, and FRER strips can be particularly sensitive. What looks like a positive on one brand may be a high-but-not-peak reading on another.
The human eye struggles with subtle line differences — especially under different lighting conditions, at different times of day, and when comparing strips from different batches. This is exactly what ClearLine AI is built for: consistent, objective analysis of your OPK strip photos so you never have to guess whether a line is dark enough.
When Should You Start Testing?
When to start OPK testing depends on your cycle length. For a 28-day cycle, ovulation typically occurs around day 14, so begin testing around day 10. For shorter cycles (21–24 days), start as early as day 8. For longer cycles (32–35 days), you can start around day 12–14.
A useful rule of thumb: start testing 5 days before your expected ovulation day. Expected ovulation is calculated as your cycle length minus 14 days (based on the typical 14-day luteal phase). Continue testing daily until you detect your surge and confirm it is fading.
If you are unsure of your cycle length, track for one full cycle using calendar estimates, then refine your testing start date for the next cycle. Combining OPK testing with our fertility window calculator gives you a strong starting framework.
OPK Surge Timing by Cycle Length
Use this table to plan when to start OPK testing based on your average cycle length. “Day” counts from the first day of your period (day 1 = first day of full flow).
| Cycle Length | Expected Ovulation | Start Testing | Surge Window |
|---|---|---|---|
| 21 days | Day 7 | Day 8 | Day 5–7 |
| 22 days | Day 8 | Day 8 | Day 6–8 |
| 23 days | Day 9 | Day 8 | Day 7–9 |
| 24 days | Day 10 | Day 8 | Day 8–10 |
| 25 days | Day 11 | Day 8 | Day 9–11 |
| 26 days | Day 12 | Day 8 | Day 10–12 |
| 27 days | Day 13 | Day 8 | Day 11–13 |
| 28 days | Day 14 | Day 9 | Day 12–14 |
| 29 days | Day 15 | Day 10 | Day 13–15 |
| 30 days | Day 16 | Day 11 | Day 14–16 |
| 31 days | Day 17 | Day 12 | Day 15–17 |
| 32 days | Day 18 | Day 13 | Day 16–18 |
| 33 days | Day 19 | Day 14 | Day 17–19 |
| 34 days | Day 20 | Day 15 | Day 18–20 |
| 35 days | Day 21 | Day 16 | Day 19–21 |
Frequently Asked Questions
Can you miss your LH surge if you only test once a day?
Yes — LH surges can be brief, sometimes lasting only 12–24 hours. If you test once daily at the wrong time of day, you may miss the peak entirely. Many fertility specialists recommend testing twice daily (morning and evening) once you see lines starting to darken. The surge often begins in the morning and peaks by evening, but timing varies between individuals.
Why is my OPK line getting darker but not positive?
A darkening test line means your LH is rising, but a positive OPK requires the test line to be equal to or darker than the control line. The progression from faint to dark can take 1–3 days. Keep testing — you are likely approaching your surge. If lines stall at nearly-positive for several days without reaching a true positive, consider testing at a different time of day or trying a different brand.
How many days after LH surge do you ovulate?
Ovulation typically occurs 12–36 hours after the LH surge peaks. Most people ovulate within 24 hours of their peak OPK reading. This is why the day of and day after your peak OPK are considered your highest fertility days. Once the test line starts fading, ovulation has likely already occurred or is imminent.
Can LH surge last 5 days?
A typical LH surge lasts 24–48 hours, with peak LH occurring in a narrow window. However, some people experience an extended rise where lines stay dark for 3–5 days. This can happen with PCOS, perimenopause, or certain medications. If your OPK stays positive for more than 3 days, consult your healthcare provider to rule out underlying conditions.
Is a flashing smiley the same as a peak OPK?
On Clearblue Advanced Digital OPKs, a flashing smiley indicates high fertility (rising LH) but is not the same as peak. A solid smiley face means peak fertility has been detected — this is equivalent to a traditional positive OPK. The flashing smiley can appear for 1–5 days before the solid smiley, so keep testing until you see the solid face.
What if I never get a positive OPK?
If you never get a positive OPK despite testing correctly, possible reasons include: testing too early or too late in your cycle, ovulating earlier or later than expected, diluted urine from excessive fluid intake, or an anovulatory cycle. Try testing twice daily, limit fluids for 2 hours before testing, and track over 2–3 cycles. If you consistently fail to detect a surge, speak with your doctor about further evaluation.
